Several off-line dirt counters have been developed. A typical one, such as the BIOTRAN.TM. (of New Brunswick Scientific) extracts measurements such as total area and total counts from two-dimensional incoming images. Recently, an off-line system PAPRICAN MICROSCANNER.TM. (of Noram Quality Control and Research Equipment Ltd.) image analyzer applies the medium-pass filter as well as visual impact principles to detect and measure dirt speck size of two-dimensional images. The INTEC.TM. (of Intec Corp.) on-line dirt counter uses a laser light source and fiber optic receivers. It can detect dirt specks as small as 0.05 mm.sup.2. However, this device is not designed to measure dirt speck size using visual impact principles similar to human judgment as defined in TAPPI (Technical Association of the Pulp and Paper Industry) count.
There are also issued patents of interest which deal with the detection of flaws on surfaces. U.S. Pat. No. 4,172,666, Clarke, Oct. 30, 1979 distinguishes between different types of faults detected on a moving web. A defect detector scans the surface of the web with a laser and views the reflected light with a view or several detectors at various angles. Sums and differences between signals are used to discern which defects scatter light and which absorb it. The signal processing is not specifically related to the emulation of visual inspection as is the present invention.
U.S. Pat. Nos. 4,794,264 and 4,794,265, Quackenbos et al, Dec. 27, 1988, relate to an apparatus and method for uniquely detecting pits on a smooth surface. The moving surface is illuminated with a point source of laser light through a pair of beam splitters. Flaws and pits reflect specularly with different patterns. An annular mask shields one detector to view the neighborhood of a focal point and a window on the other detector constrains the signal to the central focus.
The invention in the above patents uses a beam splitter to view the same spot with a pair of detectors and to difference a central spot signal with a wider surround. However, Quackenbos et al illuminate the sample with a laser through the same beam splitter in order to observe glossy specular reflectance but does not illuminate diffusely outside the beam splitter to observe the fuse reflectance. Further, Quackenbos et al use a single detector in each beam rather than a linear array of detectors, and Quackenbos et al mask the two detectors to see only their intended domains of view rather than having the two beams differ only in sharpness of focus. Finally, Quackenbos et al use the difference of detector signals to distinguish flaws from pits but does not have a system wherein the signal differences correspond to a sharpening operation.
U.S. Pat. No. 4,740,079, Koizumi et al, Apr. 26, 1988, teaches a method and apparatus for detecting foreign substances, e.g. dirt specks or smudges, on the glossy surface of a semi-conductor chip which already contains delivered etches of the circuit. In the Koizumi et al patent, a circuit component is illuminated at near grazing incidence with a laser or a pair of lasers as the chip is physically moved past the detection configuration. A beam splitter divides the View of the surface of the chip in two identical beams with different polarization. Each beam is viewed at the same focus by a linear array of detectors. The difference signal corresponds to the diffused scattering from dirt on the chip. However, Koizumi et al does not calculate the integrated optical density as a visual impact parameter.
U.S. Pat. No. 4,724,481, Nishioka, Feb. 9, 1988, teaches a flaw detector for detecting a flaw in a sheet. The apparatus includes an array camera or series of array cameras which view a moving sheet. A circuit employs an electronic shading correction to enhance the signal to noise ratio. A memory associated with each photodetection element is used to maintain a reference level for that element. The novelty of this patent appears to reside in the algorithm of shading correction and data reduction. However, the Nishioka invention cannot be used on paper sheets with realistic reflectance nonuniformity associated with formation.
U.S. Pat. No. 4,237,539, Piovoso et al, Dec. 2, 1980, teaches an on-line web inspection system which includes a transverse web scanning means. X-ray film is made at high speeds and must be inspected for defects at speeds of up to 900 feet per minute. In the '539 patent, a flying spot crisscrosses crosses the sheet with a scan repeat of 1.5 mm. This requires three scans per millisecond. A sophisticated processing board in the host computer processes the signal and sorts features with sharp edges by size. In the '539 patent, a scanning laser is used rather than a diode array. In addition, the '539 patent does not use a board level processing of the signal to extract and sort the images of the defects. Instead, sorting is accomplished by area and by the sharpness of the edge of the feature, for example, rather than by integration of the signal strength within the bounds of a spot.
A system for detecting and classifying flaws on metallic surfaces is taught in U.S. Pat. No. 4,253,768, Yaroshuk et al, Mar. 3, 1981. In the '768 patent, a scanning laser illuminates a moving surface of, for example, a pipe, which is viewed by at least two detectors. The reflectance angles for the detectors are chosen so that most of the light reflects into the first detector when there are no defects with a low but finite intensity reaching the other detectors. The other detectors are positioned to receive increased intensity when the laser beam traverses a defect. The average signal serves as a base line for comparisons. This system has, of course, very little in common with systems for detecting dirt specks on a sheet of paper, and especially the system herein.
A particle detection method and system, for scanning the surface of a semi-conducting wafer, is taught in U.S. Pat. No. 4,766,324, Saadat et al, Aug. 23, 1988. The surfaces scanned by a laser beam, and the scattered light indicates the presence of a dirt speck. The central idea of this patent is a procedure for identification of dirt that was already present before the current processing step, and to distinguish this from dirt added by the most recent step. Once again, this has little in common with a dirt counter for measuring dirt speck sizes on a web of moving paper.
U.S. Pat. No. 4,665,317, Ferriere et al, May 12, 1987, teaches a process and equipment for sensing surface defects on a moving strip of rolled metal. Successive video images of a moving sheet of the metal are presented to a computer memory as a continuous representation of the surface. Two-dimensional digital filters are used for contour detection and the edge detection of defects. The method makes successive demands upon the host computer although the algorithms are quite general. This system bears, again, no resemblance to the system of the present application which does not use two-dimensional filters.